1. Field of the Invention:
A method for implementing an adaptive channel estimator, a prefiltering arrangement for implementing the adaptive channel estimator and a base station, in which an input signal of the channel estimator is adapted.
2. Description of the Related Art:
In radio receivers channel estimators are used for measuring the state of a radio channel. Typically, state information is required for implementing coherent detection, and additionally, it can be utilized in measuring a signal-to-noise ratio (SNR), a signal-to-interference ratio and in implementing various channel equalizers so as to remove phase distortion or inter-symbol interference (ISI). The inter-symbol interference results from linear and non-linear distortions caused to the signal in the radio channel. Inter-symbol interference is produced in band-limited channels, when the pulse form in use spreads to adjacent pulse time slots. The problem is considerable, especially at high transmission rates in multimedia services or in data transmission applications. There is a plurality of equalizers of different types, such as a decision feedback equalizer (DFE) or a maximum likelihood (ML) equalizer and a maximum likelihood sequence estimation (MLSE) equalizer based on the Viterbi algorithm. In practice, the channel equalizers are generally implemented by means of various filter structures.
Variations in user movement rates and movement in general make it difficult to design equalizers in cellular radio systems. Movement of a radio transmitter causes Doppler distortion, i.e. frequency shifts and changes in a radio channel, for instance, because of variations in terrain. A channel estimator determines a channel impulse response by means of measurements. The channel estimator is also typically implemented by means of filter structures.
If the radio transmitter moves slowly, the channel also changes slowly, whereby the channel coherence time is long and several successive channel measurements have high correlation. Thus, it is possible to use a long channel filter, which utilizes the interdependency of the measurements to improve the accuracy of the estimate. If the radio transmitter moves fast, the correlation of the successive measurements is lower and it decreases faster than in the case of slow movement. The channel coherence time is thus short. In general terms, it could be stated that the higher the speed of the transmitter, the shorter the channel estimate filter should be. This poses problems, because the filter length must be changed as the movement rate changes. In prior art this is solved by using a long filter and by setting a necessary number of taps to zero. In view of the resource management this solution is not efficient. In addition, in many cases the great number of taps increases quantization noise.